The roofs of coal mine shafts require support during a mining operation. This support is provided by roof bolts which are anchored into the rock strata found above the coal seam. In order to attach the roof bolts to the roof of a coal mine, many holes must be drilled into the rock strata and must be spaced closely enough to provide a strong, safe roof in the mine.
The bits or inserts used to drill such holes have radially extending and axially inclined cutting edges formed by intersecting leading and frontal faces of the bit. The leading face is that face which is most closely parallel to the axis of rotation of the bit. The frontal face is that face which is inclined at an acute angle to a radial line normal to the axis of rotation of the bit. The cutting edges of the bits or inserts described above are designed to be sharp so that the drills might be effectively used in the coal or stone material. The bits or inserts must be capable of resisting wear, fracture, and the abrasive action of the chips from the material being drilled. When such drill bits are power driven by high-thrust, high-torque drilling machines, the rate at which the holes can be drilled increases, but the wear experienced by the bits or inserts also increases.
The speed with which holes can be drilled, the maintenance of this penetration rate, and the wear and fracture resistance of the tools are important factors in such drilling operations. Therefore, improvement in any of these factors is desirable, and has to some degree been achieved by changing the composition of the bit or insert material, usually a cemented carbide, by adjusting the carbide grain size, or by changing the bit or insert geometry.
Examples of changes in bit or insert geometry may be found in U.S. Pat. Nos. 4,489,796, 4,527,638, and 4,342,368. U.S. Pat. Nos. 4,489,796 and 4,527,638, both issued to Sanchez et al. describe inserts in which the upper, outside corners have a radius of curvature of 1/16 inch, U.S. Pat. No. 4,527,638 also describing a 30.degree. frontal face relief angle between the top cutting edge and the top trailing edge. This relief angle remains constant from the center to the radially outermost ends of the insert. U.S. Pat. No. 4,342,368, issued to Denman, describes a rotary drill bit, or drill tip for use in such a bit having both a relief angle and an angle of inclination of the leading face which vary along the radius of the bit. The included angle at the cutting edge, defined by the intersection of the frontal face and the leading face, remains constant from the center to the outermost edge of the bit.
The present invention provides mine tool roof bits, inserts, and the like which permit mine roof drilling at high penetration rates, good maintenance of the penetration rates, and longer tool life.